Heat and Mass Transfer in U-Bend of a Pneumatic Conveying Dryer

Abstract

Computational fluid dynamics (CFD) modelling with the Eularian–Eularian formulation is used to describe the drying phenomena occurring in pneumatic conveying drying particularly in the U-bend. Two user defined scalar equations (UDSs) are inserted into the solution to take into account the solid particle moisture content and the mass fraction of water in the gas phase, respectively. The drying kinetics cover the two periods of drying: the constant drying rate and the falling drying rate. The investigation emphasized influences of the parameters (solid loading ratio, gas velocity, bend radius ratio, feed moisture content and geometry arrangement of a U-bend) on the drying rate and moisture content. A validation of the drying model is accomplished by comparing calculation results with experimental data. In general, the drying rate decreases along the axial direction of pipe due to the decrease in driving force (the air humidity increases). The mass-weighted average drying rate shows a slight decrease in and just after the U-bend, which is mainly due to the high accumulation of solid particles at the outer U-bend wall. Solid particles disperse and the slip velocities are high in the area after the U-bend. This causes the mass-weighted average drying rate to slightly increase.